Aperture assembly for electron beam device



Nov. 25, 1969 K. H. LOEFF'LER 3,

APERTURE ASSEMBLY FOR ELECTRON BEAM DEVICE Filed June so, 1967 2Sheets-Sheet 1 HEATING CURRENT TURNED 0N SPUT SIZE -HEATING CURRENTTURNED OFF T [ME (MINUTES) FIG. 5

INVENTOR KARL H. LOEFFLER ATTORNEY F IG.2

Nov. 25, 1969 K. H. LOEFFLER 3,480,817

APERTURE ASSEMBLY FOR ELECTRON BEAM DEVICE Filed June 30, 1967 2Sheets-Sheet 2 United States Patent 3,480,817 APERTURE ASSEMBLY FORELECTRON BEAM DEVICE Karl H. Loefiler, San Jose, Calif., assignor toInternational Business Machines Corporation, Armonk, N.Y.,

a corporation of New York Filed June 30, 1967, Ser. No. 650,397 Int. Cl.H01j 29/46 US. Cl. 313-82 7 Claims ABSTRACT OF THE DISCLOSURE Anelectron beam generating device utilizing heated aperture plates in aremovable heated aperture assembly and incorporating a beam-sensingsystem for detecting the beam condition 'by sensing beam current flowfrom each of the electrically isolated aperture plates.

CROSS-REFERENCES TO RELATED APPLICATIONS This invention realtes to anElectron Optical Unit similar to that described in the patentapplication Ser. No. 575,731, filed Aug. 29, 1966 with Karl Loeflle-r etal. as inventors.

BACKGROUND OF THE INVENTION Field of the invention This inventionrelates to electron beam devices and more particularly to such devicesespecially adapted for data recording.

Description of the prior art In electron beam generating devices,commonly called columns, one of the primary maintenance problems hasbeen caused by contamination buildup on the beam-exposed parts. Whilethe contamination buildup occurs on all beam-exposed parts, it isgreatest on those parts which directly intercept portions of the beam.The one such part where contamination is the greatest problem is theaperture-forming plate. Since the buildup is usually non-conductive, acharge will collect in the contaminant as the beam strikes thereafter toset up random electric fields which deflect, distort or, otherwiseadversely affect the beam. Since the aperture plates are directlyexposed to and are struck repeatedly by the beam, the charge is built upfaster. Because of the location of the plate in the immediate proximityof the beam, the resulting electric field has a very adverse affect onthe beam operation. Additionally, the buildup may become so pronouncedas actually to reduce the cross-section of the plate aperture opening.

In the usual case, the contamination is caused by organic matter beingpresent within the vacuum cavity of the electron beam device. Thegeneral source for this organic material are hydrocarbons used in thepump and on the seals of the vacuum system. To a lesser degree,contaminants can originate from fingerprints or the presence of dust,etc. being present on the column parts before assembly. In the instanceof the organic substances, molecules thereof may come to rest on theaperture plates and, in the absense of the electron beam, will build uponly to a thickness of a single layer due to the thermal equilibriumcondition within the column. However, if the beam strikes the layer, thematter can become chemically bound to the plate to permit otheroverlayers to form. These polymerized layers of hydrocarbons and othermatter substantially are non-conductive and serve to retain theelectrical charge resulting from being struck by the beam thereafter tocreate electric fields which unpredictably defleet the beam. Because ofthese uncontrolled deflections of the beam or parts of it, the columnceases to function in the manner desired.

It has been known in the past that the formation rate of this organicbuildup on the beam-exposed parts can be reduced in order of magnitudeby heating the parts. The resulting higher thermal agitation of themolecules lowers the time during which they rest on the plate. In thismanner, the possibility of the molecules being struck by the beam andbeing caused to adhere to the part on which they temporarily come torest is reduced. For example, it has been found that by heating theaperture plates to a temperature of 250 C. or higher greatly extends theoperating life of such columns by slowing the contamination buildup byseveral orders of magnitude.

However, the fact must be faced that regardless of the precautions takenin designing, assemhlying and operating the column, there usually willbe some contamination buildup. Thus, after steps are taken to heat thebeam-exposed parts, the life of the column can be extended more byreducing the effects of contaminants on the operation of the column. Anelectric charge will collect on a con taminant buildup only if thebuildup is non-conductive. By rendering the buildup conductive in somemanner, the charge will be dissipated substantially as fast as it isformed. For this purpose, the plates are heated to not only slow thebuildup rate, but also to lower the resistance to current flow forhastening the dissipation of the charge collected by the plate.

Additionally, where hydrocarbons from the lubricants of the vacuumsystem form the contaminants, the beam in striking the buildupeventually will carbonize the layers to render them electricallyconductive. Because of this desirable possibility, lubricants other thanhydrocarbons usually are not used in electron beam columns. Withefficient heating of the plates, it is possible to slow the buildup to arate nearer that of the carbonization process to further extend theuseful life of the column. Naturally, with the carbonization process,the layers become electrically conductive to carry away immediately anylocal electrical charge formed thereon.

Additionally, it is desirable to sense not only whether conditions muchas those heretofore described exist preventing the beam from strikingthe target, but if not, where the beam is being blocked or otherwisecease to exist. If such information is known, repair of thebeamgenerating device can be effected more efficiently. Otherwise whenthe beam is used to record on such materials as light sensitive film, itcannot be determined until after the film is developed whether theactual data recording is being achieved. By this time, much data can belost if not recorded by the beam. There can be many reasons for beamfailure, many of which are caused by devices located external of thecolumn in addition to the contamination of the internally-positionedaperture plates as previously described. Thus, it is desirableimmediately upon failure to determine whether the problem can becorrected by checking the external controls, etc. of the device, orwhether the column must be disassembled entirely.

One element normally struck constantly by portions of the beam, andtherefore useful in detecting the beam presence, is the aperture plateof the column. However, the need to heat the plates to preventcontamination greatly complicates the using of the plates as sensingpoints for deflecting the beam current to signal the proper operation ofthe beam. For instance, leakage current from the heating means can bemany times the magnitude of the beam current being sensed. In the past,the plates have been heated by various means to include movement of theaperture plate into proximity with a radiant heating element positionedto one side of the beam axis such as described in the US. Patent3,038,993, Masuda, entitled Aperture System for Electron OpticalInstrument. Another method for heating the aperture plates is describedin US. Patent 2,898,467, Von Ardenne, entitled Electron Oscillograph. Inthis patent, electric current is passed directly through each apertureplate with the heating thereof being accomplished by the heat generatedbecause of the resistance to current flow of the aperture plate itself.Obviously, it is diflicult if not impossible to detect the transmissionof a small portion of the beam current if a relatively large heatingcurrent is being passed through the plate. It is also difficult toprovide an aperture plate mounted for repeated movement in such aprecision instrument.

The primary object of this invention is to sense accurately theoperating condition of an electron beam device.

A further object of this invention is to sense at the various apertureplates positioned along a column the operating condition of the beamwhile heating the aperture plates to a suflicient temperature to limitthe contamination thereof.

A further object of this invention is to provide an improvedelectrically-isolated aperture plate assembly for an electron beamdevice.

SUMMARY OF THE INVENTION A beam sensing system for an electron beamdevice for detecting the operating condition of the beam at varyingsensing points along the beam to include sensing the presence of thebeam at an improved electricallyisolated aperture-forming plate whichcooperates with the lens assembly for forming the beam, whichapertureforming plate additionally is heated to limit and renderharmless the contamination thereof.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of a preferred embodiment of the invention, as illustratedin the accompanying drawings.

DESCRIPTION OF THE DRAWINGS FIGURE 1 is a perspective view of anelectron beam column incorporating the subject invention with thebeam-sensing system being illustrated in schematic form;

FIGURE 2 is a partial cross-sectional view of the non-magnetic sleeveand contact assembly of the column;

FIGURE 3 is an enlarged cross-sectional view of one of the apertureplate assemblies;

FIGURE 4 is an enlarged end view partially cut away of the apertureplate assembly taken along the lines 44 of FIGURE 3; and

FIGURE 5 shows graphically the variations in spot size of the column asthe aperture plate is heated and not heated.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION In FIGURE 1 isshown an electron beam column 8 representing one type of beam generatingdevice in which the subject invention can be employed. In the presentinstance, the column is adapted for use in data recording wherein thebeam is directed onto a target or memory element 9 for recording animage. The column comprises an elongated tubular housing 10 with anassociated cathode assembly 11 supported at one of the column andserving as an electron sourcef or emitting a beam of electrons of apreselected magnitude or intensity for passage along a column axis 13.

The electron beam is focused to a small spot size by being passedthrough the magnetic fields of axiallyspaced electromagnetic lenses 14,15 and 16 positioned in spaced relationship along the axis 13 and withinthe housing 10. Each of these lenses includes a pair of polepieces 17and 18 which transmit the magnetic flux generated in the respectiveelectrical coils 15a, 16a and 17a to a point closely adjacent the beamaxis 13. A nonmagnetic spacer 19 in each lens maintains the ends of thepolepieces adjacent the axis in axially-spaced relationship. The lenses15 and 16 also include polepiece extensions 20 and 21 separated by anon-magnetic spacer 22 and held in a non-magnetic cylinder 23, whichextensions receive the magnetic flux of the respective lenses andcooperate to form the lens magnetic gap at a position closely adjacentto the beam axis 13.

By a proper energization of the coils of each of the lenses 14, 15 and16, a magnetic field is formed which deflects the electrons of the beamback towards the axis 13 for focusing the beam to a small spot size. Thebeam thereafter is passed through aperture assemblies 24a, 24b and 240positioned downstream of the respective lenses. Each aperture assemblyincludes an aperture plate 27a, 27b and 27c, respectively, in which isformed a small aperture 28a, 28b and 280, at a position coinciding withthe beam axis. By first passing the beam through the magnetic field ofeach lens, and subsequently through the cooperating aperture assembly,the fringe electrons of the beam are intercepted and the beam is formedto a very small cross-sectional diameter or spot size suitable forwriting data onto the target 9 at a very high density. The beam isfocused in the plane of the target 9 by energizing a focusing coil 30positioned adjacent the lens 16. By properly adjusting the magnitude ofelectric current supplied to this focusing coil (from a controllablecurrent source not shown), the focal power of lens 17 is varied foradjusting the focal point of the beam to coincide with the plane of thetarget. Additionally, an annular-shaped deflection coil 31 is providedwhich, when energized, serves to deflect and scan the beam across thememory element for recording the data thereon.

To modulate the beam in response to the data being recorded, a pair ofelectrostatic deflecting plates 32 and 33 are positioned one to eachside of the axis 13. By energizing these plates to opposing potentials,the beam is deflected sufficiently to become misaligned with theaperture 270 of the downstream-positioned aperture assembly 240 therebyeffectively shutting off the beam. A more complete description of thiscolumn can be obtained by reference to the US. patent application Ser.No. 575,731 heretofore identified.

From the foregoing it can be understood that proper functioning of theelectron beam column depends upon close control of the beam size andposition as it is formed within the column. It is not unusual to formthe beam to a spot size of a few microns thereby requiring that theapertures 28 in the aperture plates 27 also be of only a few microns indiameter. Thus, any contamination buildup on the aperture plate in themanner previously described can result in complete or partial failure ofthe column to function properly. Furthermore, at the high rate at whichthe column can record data, it is imperative that immediate warning begiven in the event of a malfunction of the device so that the recordingprocess can be stopped.

In accordance with the present invention, beam-sensing elements arepositioned along the beam axis which serve to generate an immediatesignal indicative of the passage of the beam to the location of thesensor thereby indicating the occurrence of and the approximate positionof the malfunction if such occurs. The signal from each sensor is fed toa control which can be programmed either manually or automatically toindicate continuously the passage of the beam to each sensing pointthereby giving a constant monitoring of the proper operation of the beamand, in the alternative, indicating between which points the beam ceasesto pass thereby giving an immediate indication of the type ofmalfunction existing in the column.

Accordingly, as shown in FIGURE 1, the beam-sensing system comprises aseries of electrical conductors 33 extending from a beam control 34 to aseries of resistors 35 connected at one end to ground and at the otherend to various sensing elements along the electron beam column byconductors 36. Thus, the elements are positioned to intercept portionsof the beam such that any beam current striking the elements istransmitted through the conductor 36 and a resistor 35 to ground. By useof the conductors 33 extending to the beam current control which isitself grounded, the voltage drop across each of the resistors 35 isdetected to indicate the bleeding off of the fringe portions of the beamcurrent thereby signalling the passage of the electron beam past thatsensing point. The control 34 can take many forms such as a meterindicator or a computer programmed control operative to print out theresults of a checking sequence followed automatically in the event of abeam failure.

To detect the passage of the electron beam to the target 9, one sensingelement includes a diode 37 positioned behind the target which generatesa current flow passing through the connected resistor 37a indicative ofthe presence of the beam striking the diode. Another type of sensingelement positioned along the beam column axis is comprised of theaperture plates 27 of each aperture plate assembly. Shown in FIGURES 3and 4 is the aperture assembly 2411 on which is mounted the apertureplate 271:. To detect the passage of beam current through the aperture28a, a conductor 38 connects with the aperture plate and extends to oneof three spring contacts 39 held by the screws 40 threaded into aninsulating plate 41 mounted on a body 42 of the aperture assembly. Thecontacts 39 are spaced radially about the body 42. The plate 27a iselectrically insulated from the body of the assembly in being removablymounted on a disk 44 mounted on a sleeve 45 which, in turn, is removablyheld in a rigid position by interfitting within a center opening 46 ofan electrical insulating ceramic disk 47 and having a nut 48 threadedthereon. Thus, any beam current striking the plate 27a will betransmitted directly to the contact 39.

As the aperture assembly is placed into the non-magnetic cylinder 23during assembly of the column, the spring contact 39 comes into abuttingrelationship with a stationary conductor 49 (FIGURE 2) which is heldwithin an insulating sleeve 50 in an opening 51 in the side of thecylinder 23. Thus, contact is made between each aperture plate and theexterior of the cylinder by the conduc; tor 49 which, in turn, isconnected with the conductor 36 leading to the beam check control.

In operation, the beam passes along the beam axis and a fringe portionof the beam strikes the aperture plate 27a which plate is electricallyisolated from the rest of the system in being held by the insulatingdisk 47. Therefore, the only exit for the beam current intercepted bythe plate is through the conductor 38, the abutting contacts 39 and 49and the conductor 36 through the resistor 35 to ground. The flow ofcurrent causes a voltage drop across the resistor 35, which voltage dropis detected through the conductor 33 thereby providing a signal to thebeam control 34 that the electron beam has reached that sense pointformed by the aperture plate 27a. The same structure is used in theother aperture assemblies to provide identical sensing points for thebeam.

In accordance with another feature of the invention, eachelectrically-isolated aperture plate is heated to prevent contaminationbuildup thereon which is one of the primary causes for failure of theelectron beam column. To accomplish this, the aperture plate 27a (FIGURE3) is mounted on the disk 44 held by the sleeve 45 which preferably ismade of a heat conducting material such as molybdenum. The plate is heldon the disk by screws 52. The sleeve is mounted by passage of theelongated portion 45a thereof through the opening 46 in the heatinsulating' ceramic disk 47 mounted to the body 42 by screws 54. In thismanner, the aperture plate and heat conducting sleeve are both heat andelectrically insulated from the body of the assembly.

In heat conducting relationship with the sleeve 45 are mounted a pair ofheating coils 55 within an annular heat conducting form 56. These coilsare supplied with electric current through a conductor 57 leading to asecond contact 39a (FIGURE 4) similar in structure to the first contact39 and mounted at an angularly displaced position on another insulatingblock 41a. By supplying electric current to the heating coils throughanother conductor 49 mounted to abut the second contact, the heatconducting form 56 is heated which, in turn, conducts heat onto thesleeve 45. The heated sleeve 45 then radiates heat to the disk 44 andthe aperture plate 27a by means of the large surface areas positionedopposite each other. In this manner, the aperture plates of eachaperture assembly are heated to approximately 300 C. to limit thecontamination thereof. Additionally, conductive heating of the adjacentparts of the column is prevented to a large degree, which heating mightotherwise adversely effect such parts of the column as the energizingcoils and spacers of the lens assemblies.

In FIGURE 5 is illustrated the eifects on the beam of the contaminationbuildup on the aperture plate. As heretofore described, the contaminantsif non-conductive, can assume an electrical charge to carry strayelectric fields which, in turn, randomly deflect the electron beam. Thecurves illustrate four sets of measurements taken of the spot sizechange vs. time as the heating current supplied to the heating coils waschanged. Curves 58 and 59 illustrate at time zero the measured spot sizeof the electron beam at the target with the aperture plate cold. At thistime, the effects of the stray electric fields in diff-using the beamand thereby increasing the spot size are seen. As the temperature of theaperture plate increases with time measured from the moment of firstenergizing the heating coils, the spot size diminishes as these strayfields are decreased in magnitude with the heating and subsequentcarbonization of the contaminants on the aperture plate.

Curves 60 and 61 begin with the aperture plates heated to a standardoperating temperature and at time zero the heating current supplied tothe coils is interrupted. Note, that the beam spot thereafter becomesenlarged. Such enlargement is due to the diffusion of the beam becauseof the stray electric fields created with the formation and subsequentelectrical charging of contaminants on the aperture-forming plates 27.

As another feature of the invention, the aperture-forming plates areelectrically isolated from the heating coils to prevent leakage currentfrom reaching the aperture plates and causing an erroneous indication ofthe beam current detected by the plate. For this purpose, insulatingplugs 62 are extended between the sleeve 45 and the disk 44 on which theaperture-forming plates are mounted. These plugs preferably are made ofa dielectric material such as alumina and are brazed into openings 44ain the disk 44 and openings 45b in the sleeve respectively. Thus, anyleakage current transmitted through the sleeve 45 must pass throughthese high resistance plugs to reach the aperture-forming plates. Tobleed olf any such leakage current, a conductor 63 is connected to thesleeve 45 and extended to a third contact 3912 cooperating with a thirdconductor 49 connected to ground and not shown. Thus, any leakagecurrent in the sleeve is transmitted directly to ground and preventedfrom reaching the aperture plate.

Experience has also shown that as electrons strike the aperture plate27, some will pass on through and strike the disk 44 and the insulatingplug 62. Because of the conductive qualities of the disk, the electronswill be transmitted directly to the conductor 38. However, the electronsstriking the insulator 62 will not be conducted away because of theinsulating properties of the alumina. Thus, stray electrical fields canbe formed which can affect the functioning of the beam. To isolate suchfields from the beam, an extension member 64 on the sleeve 45 ispositioned between the plug and the beam. Since this extension is madeof the conductive material (molybdenum 1n the example shown) anyelectric fields will be 7 shorted to ground and shielded fromintersecting the beam path.

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

I claim:

1. A system for sensing the operating conditions of an electron beamcolumn for directing a beam onto a target, said system comprising:

a plurality of beam-sensing elements positioned along said beam axis andadapted to generate a signal indicative of the passage of the beam withone of the said sensing elements being an electrically-isolatedaperture-forming plate including means for generating a signalindicative of a portion of the beam striking said plate, and

a control for indicating the operating condition of said column actingin response to said signals generated by said sensing elements,

an electric heating means for heating said aperture plate, said heatingmeans being electrically isolated from said aperture plate.

2. A control system as defined in claim 1 including an electricalcircuit connecting with said plate for conducting away any beam currentresulting as the beam strikes the plate and including means formeasuring the conduction of said current through said circuit.

3. A control system as defined in claim 1 including a beam-sensingelement positioned at the target end of said beam to supply a signal tosaid control.

4. A control system as defined in claim 1 including electric currentinsulating means positioned between said aperture-forming plate and saidheating means and at a lower voltage potential than said plate therebyto conduct stray electric currents originating at said heating meansaway from said plate.

5. A control as defined in claim 4 wherein said electric currentinsulating means is shielded to prevent any charge thereon fromgenerating electric fields which intersect said beam axis.

6. A heated aperture assembly for use in an electron beam columncomprising:

a body member formed to interfit with said column, an aperture formingplate, a heat conductive member holding said plate, insulating meansmounting said heat conductive member on saidbody member; and an electricheating element mounted on said heat conductive member inheat-conducting relationship with said aperture-forming plate wherebysaid plate and heating element are electrically and heat insulated fromsaid body member and column, an electrical isolating means positionedbetween said heating element and said aperture plate to limit the directconduction of electric current therebetween, said isolating meansincludes an electrically conductive member at a lower voltage potentialthan said aperture-plate thereby to intercept any stray electriccurrents flowing from said electric heating element before reaching saidplate. 7. An aperture assembly as defined in claim 6 wherein saidaperture-forming plate is removably mounted on said heat conductivemember.

References Cited UNITED STATES PATENTS 3,03 8,993 6/1962 Masuda 250-4953,170,116 2/1965 Farrington 324-70 3,295,008 12/1966 Gallaro et a1.315-3 3,293,429 12/1966 Leboutet et a1 250-41.9 3,239,664 3/1966 Farrell25049.5 3,345,529 10/1967 Loefiier et al 3l384 JOHN W. HUCKERT, PrimaryExaminer B. ESTRIN, Assistant Examiner US. Cl. X.R.

